As far as I understand they require us to use unmodified thin set when using Kerdi™ boards or the like. Because they are waterproof, in case you would use modified thinset this will trap the moisture between the Kerdi and the tile. They say use unmodified mortar/thinset as it will cure or dry out properly (not sure why? Less water?)

So does the same logic (humidity in the mortar gets trapped between the wall and the substrait) applies when you use something like Redguard™?

  • 1
    The best advice is to follow the manufacturer instructions, which may depend on the surface you are applying to or what you are applying to it.
    – blarg
    Sep 3, 2021 at 15:50
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    Which manufacturer? Tiles,Substrat,Thinset or Membrane?
    – MiniMe
    Sep 3, 2021 at 15:56
  • Follow the manufacturer's instructions to the letter for the products you're using. If you're mixing brands, talk to each mfgr about usage instructions and brace yourself for "you shouldn't use our competitor's inferior product". TBH, they may void their warranty if you cross systems...
    – FreeMan
    Sep 3, 2021 at 16:31
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    Which manyfacturer??? Which prkduct?
    – MiniMe
    Sep 3, 2021 at 16:38
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    The manufacturer(s) of the product(s) you want to use. Using Kerdi board? What do their instructions say? Using Redguard? What do their instructions say? Using thinset? What does it say on the tub/box?
    – FreeMan
    Sep 3, 2021 at 17:03

1 Answer 1


The manufacturers suggestions are what you should listen to.

However... Their suggestions are often there for an "in case you messed up a little". Schluter (Kerdi) wants unmodified so that the mortar can weep to the bottom. Their whole system is about pushing water down the wall. The weak points in their system are the seams.

I am guessing that they feel the the highest point of failure is water trapped in a seam area and eventually degrading that area. So they want unmodified so that it drains better - downward of course.

Redgard and the paint on waterproofing systems have a different failure point. That is mainly the area that you didn't paint enough or good enough. So modified thinset makes sense here as it is another barrier to this "flaw".

If you do your install "perfect" the thinset choice (given it was installed perfect) will not matter, unless it is the incorrect choice for your tile type. So what do you do when you are putting together a 10'x6' double head shower that is using large porcelain tiles and a kerdi membrane? Schluter tells you unmodified, tile manufacturer tells you it has to be modified... Well we took pictures and called Schluter and they gave us the OK to use modified. The shower didn't self-destruct after a year, the world keeps going - its just thinset.

  • Horror story from JohnBridge's website johnbridge.com/vbulletin/showpost.php?p=1401192&postcount=8 "The shower in question was about a year old before the problem manifest. I cut out a section of grout at the bottom of the wall and liquid water began to run out. I opened the other side of that wall to find what I thought would be a plumbing leak. No leak. Customers left town for two weeks and the tile wall was still weeping when they returned. After consultation they elected to keep the seep rather than go for the repair (complete re-do of the shower)."
    – MiniMe
    Sep 3, 2021 at 17:49
  • So humidity could get trapped there and stay there for an year without seeing it so you might need to wait longer for your shower. I am not using kerdi, it was given as an example. I use cement board + redguard and I need to know what thinset to use over that for large tiles. I am tempted to go with this one: homedepot.ca/product/… ALso see my other thread reagarding thinset for large tiles on walls
    – MiniMe
    Sep 3, 2021 at 17:51
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    Your question is not what that thread is about. The question and horror story were about the back buttering of the tiles. He said that the install was spot butter... so maybe just 5 spots on large tiles - allowing significant pockets of air. Based on the install the water got in and then too forever to weep... That's really a totally different issue and could be an issue with modified or unmodified thinset based on tile type and if grout is sealed and how bottom is worked.
    – DMoore
    Sep 3, 2021 at 17:57
  • I am still not clear what happens with the humidity in the mortar, how does it cure if you have redguard on one side and tiles on the other side and the mortar sandwiched between the two ?
    – MiniMe
    Sep 4, 2021 at 14:54
  • Here is another mention of the same issue makeitright.ca/holmes-advice/bathroom-renovation/….
    – MiniMe
    Sep 4, 2021 at 16:22

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